It's to donate bone marrow. It's taken while you're still alive. You only donate if there is a person that you are a match for. They contact you for testing if there's a potential match. The more people who are on the list to donate the better chance of a match being found for someone in need.
There's no financial commitment, either. I never paid a dime, and they reimburse you for travel and meals during days you have to donate or get tests, or any kind of official business. Depending on the situation they might even put you up in a hotel, too. I got to stay in a pretty nice hotel, and have a fairly fancy room service dinner. And they let me keep my hotel bath robe. Which is cool.
As for the donation itself, there's two methods, that I'm aware of.
Surgically. I don't know the details, since this isn't what I did.
'Peripheral Blood Stem Cells'. Which is what I did. They give you a medication that increases the production of cells they need, which will spill over into your bloodstream. Then they take blood, the same way they would if you were donating blood, filter it in a machine, and put it back into you sans the cells that would be used to treat the patient. There's not much pain involved and most importantly it is saving someone's life.
This is something I can't emphasize enough. You will know, for certain, that your cells are going to treat a SPECIFIC person who needs them to have a chance to live. I saved someone's fucking life. I don't mean that to try and brag, the effort I had to put in to do so was pretty minimal but it's an amazing feeling to realize, whatever shit I may feel about myself, however miserable of a person I am, I saved a person's life. There is a family out there who is not one member short because of my cells. It's mindblowing.
As for your health, if you check out their website www.bethematch.org they should be able to tell you what conditions would make you ineligible. I think unless you're a cancer survivor, or have HIV, or other diseases that could be communicable through blood, you're not ruled out, but I can't say for sure.